Key Words.:

–
–
star formation

Abstract

Context:Different environmental conditions can play a crucial role in determining
final products of the star formation process and in this context, less favorable
activities of star formation are expected in the external regions of our Galaxy.

Aims:We studied the properties of the young open cluster NGC 1893 located about
12 Kpc from the galactic center, to investigate how different physical conditions
can affect the process of star formation.

Methods:By adopting a multiwavelength approach, we compiled
a catalog extending from X-rays to NIR data to derive the cluster membership.
In addition, optical and NIR photometric
properties are used to evaluate the cluster parameters.

Results:We find 415 diskless candidate members plus 1061 young stellar objects with a circumstellar disk or class II
candidate members, 125
of which are also Hα emitters. Considering the
diskless candidate members, we find that the cluster distance is
3.6±0.2 kpc and the mean interstellar reddening is E(B-V)=0.6±0.1 with evidence
of differential reddening in the whole surveyed region.

Conclusions:NGC 1893 contains a conspicuous population of pre-main sequence stars together
with the well studied main sequence cluster
population; we found a disk fraction of about 70% similar to that
found in clusters of similar age in the solar neighbour and then, despite expected unfavorable conditions for star formation,
we conclude that very rich young clusters can form also in the outer regions of our Galaxy.

1 Introduction

The sequence of events that characterize the star formation (SF) process starts from the compression
of gas in a giant molecular cloud that produces molecular cores; these collapse into protostars
evolving in pre-main sequence (PMS) objects through accretion of material and circumstellar disk formation
(zinn07).
Within this standard picture, however, different environmental properties may play a key role by changing
the physical conditions during the SF process and possibly the final products, for example the initial
mass function (IMF).

SF regions with peculiar properties are therefore crucial for a complete understanding
of the SF process since they are the natural places where different initial conditions
can lead to the formation of clusters with non-standard IMF, dynamical evolution and/or
disk fraction and evolution.
In this context, clusters in the
outer Galaxy are very interesting since they are located in regions where
surface and volume densities of atomic and molecular hydrogen are much smaller than in the inner Galaxy
(wout90),
while metal content is, on average, smaller (wils92)
and therefore higher temperatures of the cloud are expected due to lower radiative losses.
In addition, the lack of prominent spiral arms and the presence of few supernovae as external triggers for SF
are further indications of less likely SF conditions.

We have identified NGC 1893 as a very promising cluster
for this kind of investigations since it is very young (∼1-2 Myr), distant about 12 Kpc from the Galactic
Center, with known massive members and indication of a large PMS population (vall99; shar07).
This SF region, located at the center of the Aur OB2 associations, is associated with the HII region IC 410
and the two pennant nebulae, Sim 19 and Sim 130 (gaze52). It is obscured by
several dust clouds and contains at least five O-type stars (hilt66). The presence of
several O-type stars and
emission line B-type stars (marc02),
likely in the PMS phase, is a strong indication
that very recent SF events have occurred in this cluster.

Several photometric and low resolution spectroscopic
works have been devoted to this cluster since the first study by hoag61.
Many of them are, however, aimed at studying the NGC 1893 massive stars, that are in
the MS phase
(beck71; moff72; cuff73; hump78; tapi91; fitz93; mass95; marc01; marc02). Evidence
of a large population
of PMS stars in this cluster has been found by vall99 by using
near-infrared photometry,
even if their data did not allow them to distinguish the young population from
the contaminating
field stars. Only recently, shar07 presented a study about the low
mass population
in NGC 1893 based on deep optical photometry down to V∼22 and NIR 2MASS
(2mass) catalog. Several
conclusions are drawn in this work about the IMF, the dynamical evolution and
the effects of massive cluster
stars on the low mass population. However, their results are based on a
statistical subtraction of the field
star population falling in the same region and suffer from the large statistical
uncertainties
related to the membership.
In fact, the main challenge of the analysis of the low mass population in very
young
clusters is the membership derivation since PMS stars usually lie
in the same region occupied by contaminating
field stars and it is therefore crucial an accurate analysis to identify cluster
members.

We have started a project aimed at the detailed study of the star formation process in NGC 1893
and in particular the unknown low mass population:
the first results, based on
the joint Chandra-Spitzer large program The Initial
Mass Function in the Outer Galaxy: the star forming region NGC1893 (P.I. G. Micela), have been published in
cara08. In this latter paper,
a conspicuous sample
of low mass cluster members with circumstellar disk and of more evolved diskless candidate members
belonging to this region has been identified by using
Spitzer-IRAC and Chandra X-ray
data.

We present here the results on the membership and cluster parameters
based on a multiwavelength approach including
new deep optical and JHK photometry that allow us
to derive a more complete census of the low mass population in this cluster;
knowledge of individual members is used to derive cluster parameters (reddening and distance)
in a more accurate way than previously reported in the literature. Detailed studies on the
X-ray coronal properties, disk fraction and evolution and IMF will be presented in forthcoming
papers.

The present paper is organized as follow. In Sect. 2 we present the observations, the data reduction method and a description of the procedure used to derive
the photometry and the astrometry. The whole catalog is presented in
Sect. 3 while Sect. 4 includes
the comparison with previous works; Sect. 5 describes how
we identify cluster candidate members by using VRIJHK, Hα and X-rays data, while in Sect. 6
the cluster parameters are derived and a comparison with literature results is presented in Sect. 7.
In Sect. 8 individual masses and ages are derived while in Sect. 9 we present our summary and conclusions.

2 Observations, data reduction and photometry

We present here new optical and near infrared (NIR) data of the cluster,
collected from the Telescopio
Nazionale Galileo (TNG) at the Roque de los Muchachos Observatory (ORM, Canary
Islands, Spain), and from the 2.2 m telescope at the Calar Alto Observatory (Spain).

2.1 Optical data

Optical observations were acquired in service mode using two different
telescopes,
with the standard VRI and Hα photometric
filters: the Device Optimized for the LOw RESolution (DOLORES) mounted
on the TNG was used in service mode during three nights in 2007, and
the Calar Alto Faint Object Spectrograph (CAFOS), mounted on
the 2.2 m telescope in Calar Alto German-Spanish Observatory (Spain),
observed for three nights in 2007 and 2008, as detailed in
Table 1.
Standard fields were taken with both instruments to perform the required
photometric calibration.

Figure 1: Upper panel: I band combined image of the four DOLORES frames of 740 sec; the overplotted
circle indicates the CAFOS FoV.
Bottom panel: Js band combined image of the NICS frames reduced and coadded with SNAP.

Date

α (J2000)

δ (J2000)

Instrument

Filter

Exp. time[sec]

seeing

airmass

2007/09/21

5:22:27.4

33:23:46.0

DOLORES

V

10, 60, 2×500

1.0″

1.151–1.192

2007/09/21

5:22:27.5

33:23:36.9

DOLORES

R

10, 70, 700

1.0–1.1″

1.066–1.079

2007/09/21

5:22:27.4

33:23:42.5

DOLORES

I

10, 60, 2×740

0.9″

1.095–1.138

2007/09/21

5:22:27.6

33:23:33.0

DOLORES

Hα

60, 300, 2×700

0.9–1.0″

1.024–1.049

2007/10/18

5:23:11.6

33:23:34.3

DOLORES

V

10, 60, 2×500

0.8″

1.026–1.040

2007/10/18

5:23:11.9

33:23:37.1

DOLORES

R

10, 70, 700

0.8–1.0″

1.006–1.010

2007/10/18

5:23:11.8

33:23:22.0

DOLORES

I

10, 60, 2×740

0.8–1.0″

1.009–1.021

2007/10/18

5:23:12.8

33:23:47.2

DOLORES

Hα

60, 300, 2×700

0.8″

1.013–1.031

2007/11/14

5:23:06.9

33:32:29.7

DOLORES

V

10, 60, 2×500

1.0″

1.282–1.353

2007/11/14

5:23:06.8

33:32:14.6

DOLORES

R

10, 70, 700

1.0–1.1″

1.113–1.134

2007/11/14

5:23:06.7

33:32:24.5

DOLORES

I

10, 60, 2×740

0.8–0.9″

1.184–1.257

2007/11/14

5:23:06.7

33:32:08.2

DOLORES

Hα

20, 60, 300, 2×700

0.8″

1.050–1.096

2007/11/14

5:22:28.7

33:32:30.5

DOLORES

V

10, 60, 2×500

1.0″

1.004–1.007

2007/11/14

5:22:33.9

33:32:39.3

DOLORES

R

2×10, 70, 700

0.8–0.9″

1.015–1.022

2007/11/14

5:22:32.0

33:32:20.5

DOLORES

I

10, 60, 2×740

0.9–1.0″

1.004–1.010

2007/11/14

5:22:34.2

33:32:47.4

DOLORES

Hα

60, 2×300, 2×700

0.8″

1.029–1.066

2007/10/11

5:23:22.0

33:26:38.7

CAFOS

V

15, 3×500

1.4–1.6″

1.002–1.004

2007/10/11

5:23:21.5

33:26:37.4

CAFOS

Hα

15, 100, 3×500

1.6–1.8″

1.002–1.016

2008/01/05

5:23:23.3

33:26:42.4

CAFOS

V

10, 60, 2×300

2.4″

1.161–1.205

2008/01/05

5:23:23.2

33:26:45.8

CAFOS

R

10, 60, 2×300

2.3–2.6″

1.231–1.283

2008/01/05

5:23:23.3

33:26:47.3

CAFOS

I

10, 60, 3×500

1.5–1.6″

1.353–1.502

2008/01/05

5:23:23.1

33:26:38.9

CAFOS

Hα

10, 60, 500

1.4″

1.603–1.651

2008/01/09

5:23:24.2

33:26:41.6

CAFOS

Hα

15, 100, 3×500

1.2–1.7″

1.139–1.239

Table 1: Optical observations of the cluster NGC 1893

DOLORES is a focal reducer instrument equipped with a 2048×2048
square pixels CCD, with a scale of 0.252′′/px and a field of
view (FoV) of 8.6′×8.6′. A mosaic of four fields
was made, covering an approximate area of
22.2′×18.0′.
The standard VRI filters were used in the four fields, as well as
the Hα narrow band filter centered on 6658 \AA, with a Full Width
Half Maximum (FWHM) of 57 \AA. We also observed the standard field SA 98
(land92, α=6h:52m:04.1s,
δ=-00d:19m:37.9s).

CAFOS is a focal reducer instrument that works with a CCD of
2048×2048 square pixels, and has a scale of 0.53′′/px,
covering a circular FoV of 16′ diameter.
With this FoV we covered the central areas of the cluster not observed by
the four
DOLORES fields. Standard VRI filters were used, together with an Hα
filter centered at 6577 \AA with FWHM of 98 \AA. The fields used as
standard for the photometric calibration were
RU 149 (α=07h:24m:40.8s,
δ=-00d:34m:10.0s),
PG 2331 (α=23h:34m:14.1s,
δ=+05d:46m:25.9s),
and SA 95 (α= 03h:53m:32.9s,
δ=+00d:00m:48.3s). In this case we
noticed some variability in the photometric conditions of the last two
nights in Calar Alto. To improve the quality of the photometric
calibration we used also a set of local standards of the cluster,
selected during the TNG data calibration, as explained below.
A combined image of the four TNG fields in the
I band, with the shape of the CAFOS FoV over plotted, is shown in
Fig. 1.

Optical data were reduced using the standard Image Reduction and
Analysis Facility (IRAF) routines, including the bias and flatfield
subtraction. Bias correction was applied by using Zero images and
the local overscan strip in each image.
Some non-standard steps were taken due to various problems
arisen during in the reduction, with details described below.

For the DOLORES observations
a combination of sky flat-fields was used to create a master flat
field in each band, using the task flatcombine. Once the images were
corrected from flat-field, a strong illumination pattern was still
present in the images of the I band. An illumination function was
created using the flat-field (with the task mkillumflat),
and conveniently removed from images. A
pattern of interference fringing was still present in the I band
images, with no variations found in the different nights. We
constructed a fringing image by combining 15 images
in the I band of the three nights, so the stars in the different
fields are canceled. The images selected were of
short exposure (2–60 s) to reduce the number of stars in the
fields. The resulting fringing
pattern was successfully corrected using the task mkfringecor and
applied to every image in the I band.

For the CAFOS data,
dome flats were used to correct the
images from local irregularities. The singular circular shape
of the FoV required special care: the values of all the
pixels outside the FoV (easy to filter because they are well below the
background) were set to a very large number; in this way
no unreal features are created in the target images after subsequent
flat-field removal, resulting in a value well below background outside the
FoV. The remaining reduction followed the standard path.

2.2 NIR data

NIR observations were acquired in service mode at the TNG, using the
large field Near Infrared Camera Spectrometer (NICS), that is
a camera equipped with a 1024×1024 square pixels HgCdTe HAWAII infrared array,
with a scale of 0.25′′/pix and a FoV of 4.2′×4.2′(baff01).
Our observations were acquired with the
Js(1.25 μ), H(1.63 μ) and K’(2.12 μ)
filters during eight nights in 2007 and 2008;
we observed the whole field with a raster of
4×4 pointings, at each pointing we obtained a series of NINT dithered
exposures; each exposure is a repetition of a DIT (Detector Integration Time) times
NDIT (number of DIT), to avoid saturation of the background;
details are given in Table 2.

Date

Field

α (J2000)

δ (J2000)

Filter

Exp. time[sec]

NINTs

seeing

airmass

2007/10/08

10

5:22:59.1

33:25:20.2

H

600

15

0.8″

1.103

2007/10/08

10

5:22:59.0

33:25:23.3

J

510

17

0.8″

1.133

2007/10/08

10

5:22:58.8

33:25:26.6

K’

700

14

0.8″

1.178

2007/10/08

5

5:22:46.9

33:21:48.1

H

600

15

1.0″

1.003

2007/10/08

5

5:22:44.4

33:21:10.7

J

510

17

1.2″

1.004

2007/10/08

5

5:22:41.5

33:21: 1.1

K’

700

14

0.8″

1.009

2007/10/08

6

5:22:42.6

33:25:15.6

H

600

15

1.0″

1.022

2007/10/08

6

5:22:42.1

33:25:20.0

J

510

17

1.0″

1.035

2007/10/08

6

5:22:41.6

33:25:24.7

K’

700

14

1.0″

1.056

2007/10/10

11

5:22:59.3

33:29:15.7

H

600

15

0.8″

1.197

2007/10/10

11

5:22:59.1

33:29:18.6

J

510

17

1.0″

1.243

2007/10/10

11

5:22:59.0

33:29:22.6

K’

700

14

0.8″

1.314

2007/10/10

9

5:23: 0.2

33:21: 2.8

H

600

15

1.0″

1.073

2007/10/10

9

5:22:60.0

33:21: 6.0

J

510

17

1.0″

1.096

2007/10/10

9

5:22:59.8

33:21: 9.8

K’

700

14

1.0″

1.133

2007/10/19

12

5:22:59.6

33:33:59.9

H

610

16

0.8″

1.467

2007/10/19

12

5:22:59.7

33:34: 0.0

J

510

17

0.8″

1.375

2007/10/19

12

5:22:59.7

33:34: 0.0

K’

700

14

0.8″

1.313

2007/10/19

15

5:23:17.2

33:28:55.8

H

600

15

0.9″

1.186

2007/10/19

15

5:23:17.3

33:28:53.7

J

510

17

1.0″

1.144

2007/10/19

15

5:23:17.5

33:28:50.5

K’

700

14

1.0″

1.114

2007/10/19

16

5:23:19.0

33:32:58.1

H

600

15

1.0″

1.034

2007/10/19

16

5:23:19.0

33:32:58.1

J

510

17

0.9″

1.049

2007/10/19

16

5:23:18.7

33:34: 5.0

K’

710

15

0.9″

1.078

2007/10/19

7

5:22:40.7

33:30: 5.1

H

600

15

0.9″

1.571

2007/10/19

7

5:22:40.6

33:30: 5.1

J

510

17

0.9″

1.838

2007/10/19

7

5:22:40.7

33:30: 4.8

K’

700

14

0.9″

1.707

2007/12/17

1

5:22:21.9

33:21:15.0

H

600

15

1.2″

1.992

2007/12/17

1

5:22:21.9

33:21:15.0

K’

700

14

1.2″

2.272

2008/01/01

3

5:22:27.7

33:30:52.3

H

600

15

1.0″

2.047

2008/01/01

3

5:22:27.6

33:30:52.5

J

510

17

1.0″

1.882

2008/01/01

3

5:22:27.7

33:30:52.3

K’

700

14

1.2″

2.290

2008/01/11

1

5:22:23.9

33:21:53.5

J

510

17

1.2″

1.419

2008/01/11

4

5:22:23.9

33:33:53.5

J

510

17

1.5″

1.642

2008/01/13

4

5:22:30.2

33:34: 5.0

H

650

16

1.0″

1.019

2008/01/13

4

5:22:30.2

33:34: 4.9

K’

700

14

1.0″

1.034

2008/01/16

13

5:23:25.9

33:22:22.1

H

600

15

0.8″

1.151

2008/01/16

13

5:23:25.9

33:22:22.1

J

510

17

0.8″

1.196

2008/01/16

13

5:23:25.9

33:22:22.1

K’

700

14

0.9″

1.248

2008/01/16

14

5:23:24.8

33:26:43.0

H

600

15

1.0″

1.586

2008/01/16

14

5:23:24.8

33:26:42.9

J

510

17

0.8″

1.489

2008/01/16

14

5:23:18.6

33:26: 5.8

K’

760

16

0.8″

1.342

2008/01/16

2

5:22:20.4

33:25:10.2

H

138

31

1.2″

1.170

2008/01/16

2

5:22:20.4

33:25:10.2

J

540

18

1.2″

1.213

2008/01/16

2

5:22:18.4

33:25:38.5

K’

700

14

1.0″

1.047

2008/01/16

8

5:22:39.8

33:32:47.8

H

560

14

1.2″

1.006

2008/01/16

8

5:22:39.8

33:32:47.8

J

510

17

1.0″

1.011

2008/01/16

8

5:22:39.8

33:32:47.8

K’

700

14

1.0″

1.023

Table 2: Near Infrared observations of the cluster NGC 1893 from NICS; NINTs is the number
of single frames.

Raw IR images taken with NICS were reduced by using the Speedy Near-IR data
Automatic reduction Pipeline (SNAP) implemented for NICS using the ”full dither”
options. We forced the script to use an external master flat field obtained
by a median combination of single flat fields taken during the same night or the
nearest night.
After a bad pixel correction, the pipeline performs the source detection
on the sky subtracted and flat fielded images. A double iteration
is performed to obtain a coadded image for each filter
by using the offsets among the images computed by a
cross-correlation algorithm after the presence of the field
distortion is corrected.
A combined image in the Js band of the NICS images reduced and coadded with SNAP is shown in Fig. 1
(bottom panel).

2.3 Photometry and data selection

Sources detection and Point Spread Function (PSF) photometry were
made using the routines within the Fortran stand-alone
versions of the DAOPHOT II and ALLSTAR packages (ste87),
improved with the use of ALLFRAME (ste94). Instrumental
PSF photometry was corrected by using appropriate aperture
corrections derived by growth curves computed with the DAOGROW code (stet90).
Details on the photometric calibration are given in Appendix.

To perform an accurate analysis of the color-magnitude and color-color diagrams and compare them
with appropriate theoretical models we selected the optical photometric catalogs obtained from the
Dolores and Cafos images by considering only objects with photometric error smaller than a magnitude-dependent limit and with the DAOPHOT parameter
sharp smaller than 0.5 i.e. objects with brightness distribution consistent with point-like sources.
The Dolores and Cafos final catalogs include 7144 and 3222 objects, respectively.

The photometric JHK catalog obtained using NICS images
was selected by considering only objects with
magnitude errors smaller than 0.2√2 in order to have color errors smaller than 0.2.
This criterion was
adopted in order to discard false detections found in the spots due to saturated objects.
After this selection our JHK catalog contained
11181 entries including multiple items
for the objects located in the overlapping regions of the 16 fields.

To filter out the redundant
detections, we first estimated an appropriate matching radius
by computing the offset cumulative distribution of the objects in adjacent fields
within a relatively large
radius equal to 3′′ using a binsize of 0.2′′,
as shown in Fig. 2 (dotted line).

We compared this distribution with
the expected cumulative distribution of spurious identifications (dashed line in Fig. 2)
computed by using the relation

dNdr=15,16∑i=1,j=2ni∗njAij[π(r+dr)2−πr2]

(1)

where ni and nj are the total number of objects in the i-th and j-th fields,
respectively, falling in the common area Aij.
The difference between the measured offset distribution and the expected spurious one
is the true distribution (solid line in Fig. 2)
which tends to flatten at separations greater than 1.1′′.

Using
this value as the final matching radius, we included less than 10% of spurious matches and
most of the true identifications.

Therefore, we matched each star of our catalog with those of adjacent
fields and considered as the same object, those falling within 1.1′′.
The final catalog composed of single items includes
9 880 objects with K magnitude down to 17.78.

2.4 Astrometry

In order to assign celestial coordinates to the stars in the optical
catalogs, we used the 2MASS catalog (2mass)
as reference. The transformation between the two coordinate systems
was made by applying the appropriate sky projection using several tasks of
the imcoords IRAF package. To determine this
transformation we first identified through visual inspection
three stars common to the 2MASS observations and to each of our
fields. With these initial coordinates we used the task
ccxymatch to match the pixel coordinate list
of each of our fields and the celestial coordinate list
of the 2MASS catalog. The resulting single matched coordinate list
is used as input of the task
ccmap to derive the final sky projection.
This sky map is set in the master image of each field with the task
ccsetwcs, and the catalog of stars converted into celestial
coordinates using the xy2sky program of the WCSTools
UNIX package developed at the Smithsonian Astrophysical Observatory.
The assignment of celestial coordinates to the stars in the NIR catalog
was performed by using the WCSTools package.

To verify the results and estimate the astrometric accuracy, we matched
the three entire catalogs DOLORES, CAFOS and NICS, with
the 2MASS catalog, used as reference to find the astrometric
solution, and considered the offset distribution within a relatively
large value (2′′). From this distribution we subtracted the expected
distribution of spurious identifications (Eq. 1), and we obtained the distribution
of true identifications. For the matches DOLORES-2MASS,
CAFOS-2MASS and NICS-2MASS, the 1 σ corresponding values
are 0.24′′, 0.22′′ and 0.19′′, respectively,
that are then
the final accuracies for each catalog.

3 Catalog

The aim of this section is to describe the multiwavelength catalog obtained using the following data:

the new optical VRI photometry from DOLORES@TNG and CAFOS@2.2m telescope of the
Calar Alto Observatory presented here;

Figure 3: FoVs of our observations: DOLORES (solid boxes), CAFOS (solid circle),
NICS (dotted boxes) plus the Chandra-ACIS (dashed box). In the upper-left and bottom-left corners of the DOLORES (D) and NICS (N)
boxes, respectively, the corresponding field number is indicated.
The IRAC FoV is larger than all the previous ones and it is not shown here.

To match these catalogs we first
derived an appropriate matching radius for each couple of catalogs, and then we
identified the common objects by considering the nearest neighbor match within
the adopted radius.
With the aim to include as many counterparts as possible between the catalogs, we
initially computed
the offset distribution within
a radius larger than the expected astrometric accuracy of the two matched catalogs and the
distribution of the spurious identifications according to the Eq. 1.
To match the objects we use the
radius at which the true distribution (see Sect. 2.3) starts to
flatten; in all cases we have
less than 10% of spurious identifications.
We derived a matching radius of 0.7′′ between the NICS and IRAC catalogs.
The IRAC catalog includes 15 277 objects having at least one magnitude among the four
IRAC filters; 7194 sources fall within the NICS
FoV, 5020 with a counterpart in the NICS catalog. Most of the NICS objects with no IRAC counterpart are fainter than K=15,
since NICS data are deeper than IRAC observations.

Using the same method described before, we found that a matching radius of 0.7 ′′ is appropriate
to cross-match the DOLORES optical catalog with both the NICS and IRAC catalogs.
In the case of the DOLORES-NICS matches we found 5285
common objects;
using the same radius, we cross-correlated the DOLORES objects without a NICS counterpart with the IRAC catalog
and we found further 629 objects, mostly located outside the NICS images,
for a total of 5914 DOLORES optical detections having at least one NICS or IRAC
counterpart.
A radius of 0.7 ′′ was used also to cross-correlate the CAFOS and
NICS catalogs, finding 2915 common objects,
while a radius of 0.9′′ was adopted to match the CAFOS unmatched objects with the
IRAC catalog, finding further 64 objects.

The X-ray ACIS-I catalog includes 1021 sources. As mentioned in Paper I, 717
of them have a single counterpart
in the IRAC catalog while 2 sources have a double identification.
For the match
between the NICS and X-ray ACIS-I catalogs we used the same procedure adopted
in Paper I, i.e.
we considered three different
matching radii, 0.7′′, 1.5′′ and 2.5′′, in order to take into account
the degrading of the ACIS point spread function
(PSF) off axis.
We found a total of 752 X-ray sources with a single counterpart in the NICS catalog
and 10 X-ray sources with two IR counterparts
642 having already an IRAC detection.

The entire catalog includes 21 688 objects, with 9531 having only the IRAC counterpart
since the Spitzer/IRAC FoV is larger than that of the other observations.

4 Photometry comparison

We compared the photometry obtained with DOLORES and CAFOS data
in order to check the consistency of our data. We limited the comparison to V=22 since
the limiting magnitudes in the CAFOS and DOLORES catalogs are V∼22 and V∼24.3, respectively,
if we consider magnitudes with
errors smaller than 0.1.
The results of the statistical
comparison between the catalogs are shown in
Table 3 where we also show the analogous results obtained
from the
comparison with the catalog
of shar07, that is the only VRI photometric catalog available in literature.
For the latter comparison, we limited the shar07 catalog to V<20 in order to avoid effects
due to large photometric errors.
Our catalogs, obtained with CAFOS and DOLORES observations are in excellent agreement, as expected, since the
CAFOS one has been calibrated by using the DOLORES catalog. We found a good agreement also with the shar07
photometry since in all bands we have offsets smaller than 0.04 mag and the standard deviations
is always smaller than 0.2 mag. The mean offsets in the V-I color are all smaller than 0.05 mag.

A good agreement is found also from the comparison between our JHK photometry, obtained with NICS data, and
the 2MASS catalog, as shown by the corresponding statistical values given in Table 3.

Mean

σ

Median

N

DOLORES-CAFOS

ΔV

0.017

0.173

0.003

2336

”

ΔR

-0.003

0.179

-0.010

2336

”

ΔI

0.013

0.209

0.003

2336

CAFOS-Sharma et al.

ΔV

0.024

0.135

0.024

409

”

ΔR

0.044

0.153

0.034

412

”

ΔI

-0.020

0.147

-0.029

427

DOLORES-Sharma et al.

ΔV

0.044

0.156

0.027

581

”

ΔR

0.026

0.134

0.010

525

”

ΔI

-0.010

0.172

-0.037

464

NICS-2MASS

ΔJ

-0.015

0.117

-0.034

707

”

ΔH

0.026

0.118

0.006

737

”

ΔK

0.005

0.107

-0.008

729

Table 3: Comparison between our and literature optical and NIR photometric catalogs.
For each couple of catalogs, statistical values and the number (N) of
objects on which they have been computed are given.

5 Member selection

In this section we
complement our optical photometric catalog by using the literature
V and I magnitudes by shar07 for the objects detected in X-rays with Chandra/ACIS-I or
in the NIR with NICS and/or IRAC falling outside our optical fields or in saturated
or corrupted regions of the V and I images. This is only for the purpose to
find an as complete as possible
list of cluster candidate members, while in the next sections, where we derive
cluster parameters, stellar masses and ages, we will use only our photometry
to be sure to have an homogeneous photometric system.

5.1 Candidate members with circumstellar disk

Deep infrared observations in the JHK bands presented in this paper allow us to enlarge
the sample of NGC 1893 members published in Paper I, based on X-ray detection
and/or infrared photometry in the four IRAC bands.

As in dami06 and in guar07; guar09 we
selected YSOs with circumstellar disk by using reddening-independent indices
that allow us to distinguish objects reddened by the interstellar medium from
those with IR excesses due to the disk.
We have defined a generic index as

QABCD=(A−B)−E(A−B)E(C−D)×(C−D)

(2)

where (A-B) and (C-D) are two different colors while E(A-B)/E(C-D) depends only
on the adopted reddening law. In our case we used the reddening laws of muna96 and riek85,
for optical and JHK bands, respectively,
since these are those that better follow our data in the color-color diagrams;
for the IRAC bands we adopted the mean of the extinctions relative to the
K band, derived for five SFRs in flah07.
With the magnitudes available in our
multiband catalog we have defined
20 reddening independent Q-indices by using the couples of colors given in Table 4.
These indices allow us to select in a way as complete as possible, all the objects
showing excesses in a given color. For example, indices involving the (V-I) colors
should select YSOs with excess in the NIR band, if the (V-I) colors derive
from the photosphere.
However, we note that in case of accretion, the V band can be affected by the veiling
contribution; in this case, the Q index is the combination of excesses in two colors.

A-B

C-D

Qlim

A-B

C-D

Qlim

J - H

H - K

-0.017

J - K

K - [3.6]

-1.350

V - I

J - K

-0.472

J - K

K - [4.5]

-1.012

V - I

J - H

-1.047

J - K

K - [5.8]

-1.028

V - I

H - K

-0.044

J - K

K - [8.0]

-1.462

V - I

J - [3.6]

-1.008

J - H

K - [3.6]

-0.705

V - I

J - [4.5]

-0.908

J - H

K - [4.5]

-0.493

V - I

J - [5.8]

-0.968

J - H

K - [5.8]

-0.503

V - I

J - [8.0]

-1.271

J - H

K - [8.0]

-0.776

…

…

H - K

K - [3.6]

-0.644

…

…

H - K

K - [4.5]

-0.519

…

…

H - K

K - [5.8]

-0.525

…

…

H - K

K - [8.0]

-0.686

Table 4: Couples of colors used to define the reddening independent Q-indices as in
Eq. 2 and the limit
adopted to distinguish objects with normal photospheric colors from
YSOs with excesses.

For each reddening independent Q-indices we adopted a conservative limit
that chosen by considering the expected colors of main-sequence stars
(keny95) for the colors involving the VIJHK band. For the color
involving the IRAC magnitudes [3.6], [4.5], [5.8] and [8.0] we defined the
limit by considering the Q-indices of the objects that in the [3.6]-[4.5] vs. [5.8]-[8.0]
diagram have color compatible with zero within 2 σ.
By assuming these limits, given for each index in Table 4,
we consider objects with infrared excess those with
Q-index smaller within 3 σ than the adopted limit in a given index.

We note however that with the Q-index method all the selected objects
have a disk but there is an ambiguity region where stars with disk cannot
be distinguished by reddened objects and then the list of candidate members with
a disk can be incomplete.

With this criterion, we selected 984 class II YSOs; by comparing the class II YSOs with the
242 class II YSOs selected in Paper I we found 187 are common to the two samples,
only 19 are class II according to the IRAC colors
but not according to our indices, and 36 are class II according to the IRAC colors
that fall outside of the NICS FoV.

In our list of class II candidate members we found 5 of the 7 YSOs classified in Paper I
as class 0/I candidate members, for which we maintained the 0/I classification and
discarded them
from our sample of class II candidate members.
In summary, we have 1034 (984+19+36-5) class II YSOs of which
792 are new candidate members with a disk found in this work. The latter include 17
objects classified in Paper I as diskless candidate members:
we checked that the NICS photometry is of good quality
and therefore we classified these objects as class II candidate members.

The V vs. V-I diagram of the objects with circumstellar disk
is presented in Fig. 4 where all class II candidate members are
indicated by squares. The solid line is the 10 Myr solar metallicity isochrone
of sies00, drawn assuming a distance of 3.6 kpc and E(B-V)=0.6 (see Sect. 6);
from now on the theoretical models will be plotted by using
the muna96 and riek85 reddening laws for the optical and NIR bands
and the keny95 conversions to transform theoretical temperatures and luminosities in the
observational plane.
The 10 Myr isochrone is used here only to distinguish the PMS region
from the stars that have colors apparently inconsistent with a PMS nature.

We found that among the 1034 class II candidate members, there are 170 YSOs in the region below the
10 Myr isochrone. We do not believe these to be actually older than the other members, but rather,
we guess they are candidate members for which optical colors are not purely photospheric. Most of these
objects show excesses in the JHK-IRAC colors and therefore the anomalous
optical colors and/or magnitudes could be due to phenomena related to the disk presence,
such as veiling,
scattering or other effects due to the angle of inclination of the disk
(see guar10, for a discussion).

In fact, using the synthetic photometry derived from the robi06 models,
stars with circumstellar disk with age younger than 10 Myr
can have optical colors and magnitudes apparently compatible with older age.
However we cannot exclude that a small fraction of these objects could also be extragalactic sources,
as expected since the cluster NGC 1893 lies in the direction of the galactic anti-center.
We defer to a future
work an accurate analysis of the disk properties of these objects by means of
spectroscopic data.

Figure 4: V vs. V-I
diagram of all objects with optical magnitudes in our catalog.
Class II YSOs are indicated with squares.
Solid line is the 10 Myr solar metallicity isochrone
of sies00 drawn assuming a distance of 3.6 kpc and E(B-V)=0.6.

5.2 Candidate diskless members

We define as diskless candidate members all the objects
having an X-ray counterpart in the Chandra-ACIS catalog, showing any IR excess,
i.e. not belonging to the sample of class II YSOs, defined in the previous section,
and falling in the PMS region of the V vs. V-I diagram, i.e. that with colors redder than
the 10 Myr solar metallicity isochrone
of sies00 drawn assuming a distance of 3.6 kpc and E(B-V)=0.6 (see Sect. 6).

To define the PMS region, we have been guided by the bulk of objects
with X-ray counterpart, as shown in Fig. 5 which depicts the V vs. V-I
diagram of all objects with optical magnitudes in our catalog. Stars with X-ray counterpart
are indicated with
× symbols.

In the sample of diskless candidate members we also include the objects with X-ray emission that
do not show any IR excess and have already reached the MS phase at the cluster age,
i.e. those with V<15.

With the above conditions, we found 434 diskless YSOs, 85
of which are in the sample of 110
diskless YSOs found in Paper I. If we discard from the 110 diskless YSOs defined in Paper I
the 17 objects considered here as class II YSOs,
the remaining 8 diskless YSOs classified
in Paper I do not have either V or I magnitudes. By using the literature
B and V magnitudes by mass95 of these 8 objects, we found
that 5 fall in the
cluster MS region while the other 3 lie in the PMS region: we therefore
included them in our sample
of diskless cluster candidate members.

In summary, we have a total of 442 diskless YSOs.
These objects are indicated with filled circles in the V vs. V-I diagram presented in Fig. 5.

Figure 5: V vs. V-I diagram of all objects with optical magnitudes in our catalog.
X-ray detections are indicated with
× symbols while those selected as diskless YSOs are indicated with filled circles. Solid line is as in Fig. 4.

5.3 Hα emitters

In the case of stars
with accretion from circumstellar disk, the Hα line is much enlarged and therefore the
(R-Hα) index is a measure of the Hα flux in
excess.
In order to identify stars with Hα emission,
we used the R vs. R-Hα diagram shown
in Fig. 6,
where we defined the normal star limit by performing a linear fit of the R magnitudes of all diskless
YSOs (indicated with filled circles) having R-Hα<−3. Then we estimated a typical error in the
R-Hα index per bin of 0.5 magnitudes, as the average of the R-Hα errors in each bin.
We added such errors to the R-Hα derived from the linear fit to obtain the limit indicated in the
figure by the solid line. We defined as Hα emitters, those with R-Hα larger within 3 σ than this limit. Among the 269 objects that satisfy this condition we
consider as cluster Hα emitters only the 125 stars (indicated in the figure as empty circles)
for which we have an additional membership
indication, viz. 98 class II and 27 objects previously classified as diskless candidate members.

We included in the class II sample these 27 Hα emitters,
that we had classified as diskless in the previous section.
We note that this is not a contradiction in the classification but only a limit in the selection
of candidate members with disk since
there is a region in which the excesses by disks cannot
be distinguished from reddened objects.

In summary, we have a sample of 1061 class II candidate members, 125 of which are Hα emitters,
plus 415 diskless stars.
Optical photometry and classification of class II and diskless candidates members are given in
Tables 5 and 6 while new JHK photometry and Spitzer-IRAC
magnitudes from Paper I of class II and diskless candidates members are given in
Tables 7 and 8222Full Tables 5,
6, 7 and 8 are also
available in electronic form at the CDS via anonymous ftp to
cdsarc.u-strasbg.fr or via http://cdsweb.u-strasbg.fr.
\onltab6

Seq.

X ID

RA(2000)

Dec(2000)

V

R

I

Hα

Num.

[deg]

[deg]

1

55

80.6220775

33.5139642

9.045± 0.016

9.241± 0.047

0.036

2

779

80.7594680

33.5270160

11.132± 0.014

10.978± 0.017

10.817± 0.011

0.009

3

664

80.7418460

33.4434210

12.147± 0.014

11.923± 0.050

11.725± 0.016

0.009

4

276

80.6880800

33.4065610

12.269± 0.010

12.034± 0.035

11.793± 0.011

0.014

5

…

80.7888715

33.5006385

12.692± 0.022

12.103± 0.007

11.721± 0.013

0.037

6

…

80.6358343

33.5480553

12.733± 0.025

11.819± 0.006

7

887

80.7798330

33.5475840

12.938± 0.011

12.491± 0.008

12.110± 0.014

0.009

8

345

80.6968720

33.4187280

13.047± 0.011

12.628± 0.039

9

773

80.7583743

33.5198142

13.084± 0.015

12.211± 0.016

11.365± 0.021

0.007

10

…

80.7442230

33.5280269

13.105± 0.007

12.588± 0.025

12.083± 0.022

0.006

…

Notes - This table is published in its entirety in the electronic edition of
Astronony & Astrophysics. A portion is shown here for guidance regarding its form and content.

Notes - This table is published in its entirety in the electronic edition of
Astronony & Astrophysics. A portion is shown here for guidance regarding its form and content.

Table 8: NIR photometry of diskless candidate membersFigure 6: R vs. R-Hα diagram of all objects;
diskless YSOs are indicated with filled circles while Hα emitters with at least
an other membership indication are indicated as empty circles. Solid line is the limit we defined
to distinguish diskless YSOs from high Hα emitters.

6 Cluster parameters

Metallicity, interstellar reddening and cluster distance are fundamental to
compare the observed CMD with theoretical
tracks and isochrones from which we estimate masses and ages of cluster candidate members.
However, as already discussed in several papers (e.g. hill97; gull98), class II YSOs typically
experience accretion phenomena, scattering and/or reflection processes
due to the dust grains in their circumstellar disks that may alter
their photospheric colors, also in the optical bands. In such cases
comparison of observed magnitudes and colors with models can be unreliable
and the results should be interpreted with caution. In order to get around
this problem, we derive cluster parameters by considering only cluster candidate members without
circumstellar disk (class III YSOs) for which the observed colors
can be directly compared with evolutionary tracks and isochrones once the interstellar
reddening is taken into account.

6.1 Metallicity

In this work we adopt models computed for solar metallicity, since there are
ambiguous indications in the literature about the metallicity in NGC 1893. In a paper focused on
the Galactic metallicity, roll93 and roll00
derived C, N, O, Mg, Al and Si abundances of 80 B-type main sequence stars, 8 of
which are NGC 1893
members. After discarding two high rotator objects, they performed an LTE
analysis from which
they derive for NGC 1893 only a slight indication of under solar abundance.
More recently, dafl04
derived non-LTE chemical abundances in young OB stars from high-resolution
echelle spectra.
The two NGC 1893 cluster members show again a marginal indication of subsolar
metallicity, but
from the chemical analysis of 69 young stars with Galactocentric distances
between 4.7 and 13.2 kpc,
dafl04 find metallicity slopes consistent with a flattening of the
radial gradient. Even if roll00
found indication of steeper slopes, the available studies do not show strong
evidence of sub-solar metallicity and therefore, as assumed in previous studies,
we adopt a solar metallicity for NGC 1893.

6.2 Interstellar reddening

Color-color diagram using UBV photometry is the classical tool to derive
the mean interstellar reddening of a given cluster by comparing expected photospheric
colors with those observed for diskless cluster members.
Fig. 7 shows the U-B vs. B-V diagram obtained by using literature UBV photometry
by mass95 and shar07; diskless stars are indicated with filled circles. We find that most of the bluest diskless YSOs,
indicated by empty squares, are compatible with
a reddening E(B-V)=0.6±0.1 from the comparison with the
solar metallicity ZAMS of sies00, transformed into the observed colors
by using the keny95 relations.
Diskless candidate members with B-V≳0.6 can be compatible with lower reddenings, down to
E(B-V)=0.2, or with higher reddenings, at least up to E(B-V)=1.3, as shown by the ZAMS reddened with E(B-V)=0.2
and E(B-V)=1.3 (dashed lines in Fig. 7).

Figure 7: U-B vs. B-V diagram obtained by using mass95 and shar07 photometry (dots);
filled circles are diskless candidate members, squares are those compatible
with reddening 0.5<E(B−V)<0.7 based on the
expected photospheric colors by keny95 drawn at the E(B-V) values indicated on each line.
The reddening vector obtained using the muna96 reddening law is also shown.

This allows us to deduce that the cluster region is affected by a significant differential reddening and
that for the reddest objects the interstellar reddening cannot be photometrically derived.
On the contrary, it
can be unequivocally derived for the bluest objects, i.e. those indicated with squares for which
the 0.5<E(B-V)<0.7. This result has been checked by computing individual reddenings for
the 12 diskless YSOs with U-B≲0.5
for which spectral types are available (hilt66; mass95; marc02); we
found that among the bluest
objects (B-V≲0.5), all the
reddening values are compatible with E(B-V)=0.6±0.1, while among the reddest ones
(B-V≳0.5), four have E(B-V)∼0.2
and one has E(B-V)=0.68.

The mean reddening E(B-V)=0.6±0.1 we derive for NGC 1893 is consistent also in
the V vs. V-I CMD shown in Fig. 8.
In this figure, dots indicate all objects in the FoV with error in V-I smaller than 0.1 and filled
circles indicate diskless candidate cluster members. Objects indicated also with squares are those with E(B-V)
between 0.5 and 0.7, as in Fig. 7.
The thick line is
the solar metallicity ZAMS of sies00
at a distance of 3.6 kpc (see Sect. 6.3) and
reddened using E(B-V)=0.4. The value E(B-V)=0.4 is that assumed for foreground
objects that we expect to be less reddened than cluster members.
To fit the highest mass range not covered by the sies00 ZAMS, we use
the theoretical isochrone at solar metallicity of 1.5 Myr of mari08
reddened with E(B-V)=0.5, 0.6 and 0.7 at a distance of 3.6 kpc (dashed lines).
In this diagram, diskless candidate cluster members show a large spread that can be due to several reasons,
such as differential reddening, binarity and/or age spread. This is true also in the
V≲16 range where we can have objects in PMS or MS phase. However,
those with 0.5<E(B-V)<0.7, as derived from Fig. 7 (indicated
with squares)
are well fitted by the 1.5 Myr isochrone if a reddening E(B-V)=0.6±0.1 is adopted, in agreement
with the value we derived
by using the U-B vs. B-V diagram.
On the contrary, the other bright objects show a significant spread and
can be both PMS objects or very reddened MS members.
We note also that the choice of the 1.5 Myr isochrone is not critical for this result since
in this mass range, isochrones degenerate in a vertical shape branch. For this reason in the V vs. V-I
diagram, MS members cannot be used to constrain neither ages, nor the cluster distance, while
they can constrain the cluster reddening.

Our result is also consistent in the low mass range, as can be seen by using
the V-I vs J-K diagram shown in
Fig. 9, where we present
the color-color diagram of all objects;
diskless candidate members are used to estimate
the cluster reddening by considering
the locus of expected photospheric colors by
keny95; we take advantage of the fact that in this diagram the
expected photometric colors for
low mass stars are almost vertical.

In order to estimate the cluster reddening, we considered also the
distribution
of the J-K colors of diskless candidate members within fixed ranges of 0.2 magnitudes
in the interval 1.3<V-I<3.0,
and we considered, for each V-I bin, the peaks of the J-K distribution to
derive the fiducial cluster sequence,
indicated by the error bars in the bottom panel of Fig. 9.
Comparison of this fiducial
sequence with the lines allows us to confirm that the NGC 1893 mean reddening
is E(B-V)=0.6±0.1. However, since
the population of low mass stars is larger and spatially spread over the whole
region affected by a larger
differential reddening, we note that low mass candidate cluster members can individually
have E(B-V) larger or smaller values. As it will be shown later, by using
IR colors we estimate that
the reddening in this region is not larger than about E(B-V)=1.3 (AV=4 mag).

The spread of low mass candidate cluster members in the V vs. V-I diagram is not due only to differential reddening. In fact,
if we select the sample of low mass diskless candidate
members from the V-I vs J-K diagram with 0.5<E(B-V)<0.7, we find that
in the V vs. V-I diagram they show the same spread as all diskless and therefore we conclude that
the low mass star spread in the V vs. V-I diagram could
be also due to an intrinsic age spread and/or binarity.

We note that the E(B-V) value we derived both for low and high mass candidate cluster
members, is independent
of other adopted parameters such as distance, ages and metallicity.
However further spectroscopic observations should be obtained to derive
spectral types and therefore individual
reddening values.

Figure 8: V vs. V-I diagram of all the objects (dots) with errors in V-I smaller than 0.1.
Filled circles are the diskless candidate cluster members while squares indicate those with E(B-V) between 0.5 and 0.7,
according to Fig. 7.
Solid line is the solar metallicity ZAMS of sies00 at a distance of 3.6 kpc and reddening E(B-V)=0.4.
Dashed lines indicate the 1.5 Myr mari08 isochrone of solar metallicity for masses larger than 2 M⊙ at 3.6 kpc and for
E(B-V)=0.5, 0.6 and 0.7. The reddening vector obtained using the muna96 reddening laws is also shown.Figure 9: Color-color diagram of objects with error in color smaller than 0.1 (dots);
solid lines are the photospheric colors by keny95 at the three E(B-V) values indicated on each line.
Filled circles in the upper panel are the NGC 1893 candidate cluster
members without circumstellar disk while error bars in the bottom panel indicate the cluster
sequence fiducial line derive as described on the text. The reddening vector obtained using the muna96 and riek85 reddening laws is also shown.

6.3 Distance

The distance of NGC 1893 is a very uncertain parameter ranging in the literature
from 3.2 to 6.0 kpc (see Sect. 7).
Using broad band optical or NIR photometry of candidate cluster members and the isochrone fitting method,
it is very difficult to derive the cluster distance. In fact,
very young clusters such as NGC 1893 include a population of PMS stars, that lies in a ample
region of the CMD diagram, and a small population, more or less rich, depending on the cluster age, of higher mass objects already on the MS phase and therefore on a well defined locus of the CMD.
Nevertheless, the MS star distribution on the CMD is usually almost vertical and therefore quite ineffective
for an accurate distance derivation. In addition,
for clusters younger than about 15 Myr, the flattening of the isochrones between the two phases (PMS and MS)
ends on the MS at a given magnitude that is strongly dependent on the isochrone age. Unfortunately,
unless individual ages of PMS cluster members are well known,
it is very hard to constrain the cluster distance by looking for this magnitude,
even if the cluster mean reddening is fixed and a large population of high and low mass cluster members
is known.

However, in the V vs. U-B diagram, the MS population is quite well separated from the
bulk of the remaining objects that are field stars and candidate PMS cluster members.
This is evident in the upper panels of Fig. 10, obtained
by using the UBV photometry by mass95 and shar07, and where diskless
candidate cluster members selected by us are
indicated with filled circles. Among these objects, we indicate with squares those that according to the U-B vs. B-V
diagram have 0.5<E(B-V)<0.7. We use these objects here to
constrain the cluster distance by comparing again the
solar metallicity isochrone of 1.5 Myr of mari08
reddened with E(B-V)=0.6.
By taking advantage of the slightly curved shape of the isochrone in this CMD,
we find that a good fitting
is achieved if a distance of 3.6 kpc is adopted with an error of about 200 pc. With our data, we can
rule out for NGC 1893 a distance of 3200 pc or 4300 pc that are values previously given in the literature (shar07; tapi91; mass95).

A distance of 3.6 kpc for NGC 1893 is also consistent with that derived by using an independent
method
based on the nebula properties. In fact, as already done in previous works (pris05; guar07), we assume that
the nebula surrounding the cluster
obscures many background objects and allows us to see mainly foreground stars, that are
mostly in MS, with distance smaller or equal to the nebula distance. Among these latter,
those located at the nebula distance
define the blue envelope of the CMD diagram that can be fitted by the ZAMS locus. In the plausible
assumption that the nebula is located at the same distance of NGC 1893 and by considering only very high
reddened regions around the cluster (to minimize the spread due to background objects), it is possible to derive the cluster
distance by fitting the ZAMS to the blue envelope of the CMD.

By a visual inspection of our observations in the optical, JHK and Spitzer/IRAC bands, we note that
the nebula is not evenly spatially distributed, lying mostly in the western region of our FoV. For this reason,
we selected the MS stars to be used for the ZAMS isochrone fitting by using the most obscured regions.
The latter
have been selected by means of a reddening map based on the distribution of the H-K colors, as done in
dami06 by using H-K=0.1 as limit for the intrinsic color expected for background giants.
As in dami06, for the map computation we excluded all objects classified as candidate cluster members.
The AV map has been derived by using a cell size of 3.1′× 2.6′ within the FoV defined
by our NICS/JHK observations and
by the E(K-H) map, obtained as the median of H-K colors in each cell minus the expected 0.1 value.
We find that the whole region is affected by 2.2≲AV≲4, with the reddest regions being the western ones. As already mentioned, the whole region is not affected by very
high AV values and this can be also confirmed by the distribution of field stars in the J-H vs. H-K diagrams. This
is likely due to a low
absolute reddening affecting the galactic anti-center direction.

By using the resulting AV map, we selected all the objects within the regions with AV>3.8
and error in the V-I colors smaller than 0.1; the CMD diagram of these objects (indicated with dots)
is shown in the bottom panels of Fig. 10, where filled circles are diskless candidate members.
The dashed curve in the figure represents the solar metallicity ZAMS of sies00 shifted at the distance
d=3.6 kpc and using E(B-V)=0.4, that is the minimum reddening we associate to field stars according to the bright stars
in this diagram, while the solid line is the solar metallicity 1.5 Myr isochrone of mari08
for stars with mass M>1.5M⊙,
adopting the same distance and E(B-V)=0.6,
that is the mean cluster reddening we derived in the previous section.

As already mentioned, we derive the cluster distance as that value for which a good ZAMS fitting to MS stars of the blue envelope of this diagram is achieved. We note that, since the maximum
AV is about 4,
even in the most obscured regions, a little fraction of faint background objects could be visible. In fact,
as shown in Fig.10 (bottom panel), objects with V>21 do not follow the ZAMS shape, likely because this region is populated by field stars or
extragalactic objects, that are expected to be visible in
the galactic anti-center direction where the cluster NGC 1893 is located. Therefore, for the ZAMS fitting we consider only field stars with V<21; nevertheless,
since the bright population of field stars (V≲17) is poorly populated, we considered, in this range,
the candidate cluster members with V≲16 that are on the MS phase and with 0.5<E(B-V)<0.7, indicated
with squares.
We note that V≃16 is the magnitude of the Turn On, i.e. the magnitude where the PMS joins the cluster MS.
We fitted the candidate MS cluster members assuming the mean cluster reddening E(B-V)=0.6 and we
found that
a good fit is obtained both for the MS stars in the blue envelope and for the
bright candidate cluster members on MS if a distance of ∼3.6 kpc (see bottom panel of Fig.10) is used.
By taking the solar galactocentric distance of 8.5 Kpc, we deduce that NGC 1893 is located at a distance of about 12.1 Kpc
from the Galactic Center.

Figure 10: V vs. U-B diagram from the mass95 and shar07 photometry (upper panels) and
V vs. V-I diagram of all the objects with errors in V-I smaller than 0.1 and
located in the regions with AV>3.8 (bottom panels).
Filled circles are the diskless candidate cluster members; squares are those with 0.5<E(B-V)<0.7;
solid line is the 3.0 Myr isochrone of solar metallicity using E(B-V)=0.6 while
dashed line is the solar metallicity ZAMS of sies00 using E(B-V)=0.4;
both are drawn assuming a distance of 3.6 kpc.
The reddening vector obtained using the muna96 reddening laws is also shown.

7 Comparison with literature cluster parameters

The cluster distance we derived (3.6±0.2 kpc) is marginally consistent
with that recently derived by shar07 (d=3250 pc) by fitting the 4 Myr
isochrone of bert94 to the MS field stars and assuming E(B-V)=0.4,
that is the value taken for the MS field stars. This method is similar to
that used by us, since we note that
the 4 Myr isochrone of bert94 corresponds rather to the ZAMS locus, if we consider that stars
4 Myr old with masses smaller than about 2 M⊙ are yet in PMS.

We find a good agreement with the value derived by cuff73 who, using UBV magnitudes,
derived that the reddening in front of
the cluster amount to E(B-V)=0.4, while the distance is 3.6 kpc. Among the first distance values published
for NGC 1893, beck71 found d=3700 pc and
AV=1.68, corresponding to E(B-V)=0.54, by using simultaneously two color-magnitude diagrams.
moff72 derived
a distance of 3980 pc and a reddening E(B-V)=0.55 using photographic UBV photometry
and the ZAMS fitting to the unreddened CMD.
hump78 found a distance modulus of 12.52 corresponding to
a distance of about 3200 pc, by using V, B-V photometry of three O type main sequence stars of NGC 1893
in a paper devoted to study fundamental properties of the most luminous stars in our Galaxy.

A larger distance of 4300 pc has been derived by tapi91 by using Str¨omgren ubvy, Hβ and JHK photometry.
They derived spectral types and approximate luminosity classes from the m1 and c1 indices that together with literature slit spectral types are used to estimate reddening and absorption and therefore intrinsic colors and magnitudes for candidate cluster members. The corrected distance modulus is derived as the difference between the unabsorbed V magnitude and the absolute magnitude predicted for the given spectral type. A similar value, equal to 4800 pc, has been
found by fitz93, by using again Str¨omgren ubvy photometry and the theoretical ZAMS fitting.

CCD UBV photometry and multiobject fiber spectroscopy are used by mass95 to determine distance
and reddening equal to 4400 pc and E(B-V)=0.53±0.2, respectively. These values are derived by using individual reddening
from spectroscopy and appropriate spectral type-MV calibration for O and B-type stars. The derived distance
has been obtained by including only stars with inferred color excess E(B-V) between 0.41 and 0.70

vall99 use J and K photometry and UBV literature photometry to derive E(J-K) from 0.15 to 0.5, corresponding
to 0.25<E(B-V)<0.8 in the whole region they study; in addition they find two main dark regions where E(J-K) is 0.45-0.55
(or E(B-V)=0.7-0.9) that is higher than the value derived in the regions outside the dark clumps.

The most discordant distance value for NGC 1893 with respect to that found in this paper
is that derived by marc01 equal to 6000 pc; they
use photometric Str¨omgren indices (magnitude limit V≃15.9)
to compute the interstellar reddening E(b−y) of cluster members.
They find values up to E(b−y)=0.607 but they estimate the average interstellar reddening
E(b−y)=0.33±0.03
by considering only stars with E(b−y)<0.4. They use this average reddening
to derive individual magnitudes and colors (V0, (b−y)0 and c0).
By fitting an empirical ZAMS in the MV vs. c0 diagram they derive
a dereddened distance modulus of 13.9. Considering that the cluster is affected
by a significant differential reddening, we suppose that the photometric intrinsic
parameters derived by marc01, based on the average reddening could be the
origin of their overestimate of the cluster distance.

8 Masses and Ages

Assuming the cluster parameters derived by us in the previous sections, we
was able to
compute masses and ages of all objects we selected as candidate cluster members. However,
as we mentioned before, the position in the color-magnitude diagram of class II candidate members
can be influenced by effects due to the presence of circumstellar disk. Since our data
do not allow to disentangle the purely photospheric spectrum of the stars, to which
theoretical models are referred, from that in excess
due to such effects, we assume that masses and ages of diskless stars
can be accurately derived within the uncertainties due to photometric errors, reddening and to the theoretical models,
while masses and ages of class II stars should be taken with caution,
especially those of the 170 candidate members falling outside the
PMS region with ages apparently older than 10 Myr.

Masses and ages are computed by interpolating the solar metallicity
sies00 tracks
on the V vs. V-I plane, using the TRIGRID idl function. Figure 11 shows the
V vs. V-I diagram with the sies00 tracks and isochrones superposed.

For the diskless sample we find stars with masses between 0.3 and
6.7 M⊙333the upper limit is imposed by the adopted sies00 models.;
we have further 11 objects
classified as diskless candidate members with mass larger than 7 M⊙;
the median value and the standard deviation of diskless member ages are 1.4 Myr and 1.8 Myr,
respectively.
For the class II sample, if we discard the 170 stars with optical anomalous photometry,
we find stars with masses between 0.2 and 6.9 M⊙; we have further 10 objects
classified as class II candidate members with mass larger than 7 M⊙;
the median value and the standard deviation
of ages of candidate members with disk are 1.6 Myr and 2.2 Myr, respectively, with again few stars being 9-10 Myr old.
We conclude that, apart from the 170 objects with anomalous optical indices, the remaining class II stars
show mass and age distributions very similar to those of diskless YSOs as it is shown in Fig. 12 where the age distribution of diskless and class II candidate members are shown.
This reflects the similar spread observed in the V vs. V-I diagram by the two populations of candidate
members. Apparently the colors of class II candidate members are not substantially modified by phenomena
related to accretion.
The peak in the observed mass function is among 0.4 and 0.5 M⊙; however the
IMF will be computed in an other work where we will discuss the biases due to the adopted methods and where
we will compare the IMF with that of other clusters of similar age.

Figure 11: V vs. V-I diagram of all objects with optical magnitudes in our catalog (dots).
diskless and class II candidate members are indicated with filled circles and empty squares, respectively.
Lines are the sies00 tracks and isochrones
of masses and ages indicated on each line.Figure 12: Age distribution of diskless and class II candidate members in the PMS region. Class II YSOs
with anomalous blue colors are not included in this sample.

9 Summary and conclusions

We used new deep optical NIR data in the VRIJHK and Hα bands together with literature
X-ray and Spitzer-IRAC data to compile a multiband photometric catalog in the direction of the
young distant open cluster NGC 1893, located towards the galactic anti-center.
Using these data and different membership criteria, we made the most complete census of candidate members
available in the literature for this cluster. Reddening-independent indices, involving optical and/or NIR
colors, allowed us to derive a list of 1061 candidate members with a circumstellar disk, with 125 among them being
also Hα emitters; among the disk bearing
candidate members, 170 show anomalous optical magnitudes and colors that are likely
due to effects of gas and/or dust in the disk. In addition, X-ray detections and optical
magnitudes, allowed us to distinguish 415 candidate members without disk.

We used these last 415 YSOs and their photometric properties, to assess the cluster parameters,
viz. interstellar reddening and distance. Disk-less candidate members are, in fact, the most reliable objects
to be compared with theoretical tracks and isochrones, since their colors are purely photospheric
and do not suffer from additional effects due to the presence of circumstellar disks.

Bright diskless members in the U-B vs. B-V diagram, obtained from literature data, are consistent with
an interstellar reddening E(B-V)=0.6±0.1. This value is also consistent using the same bright
candidate members in the independent
V vs. V-I diagram and most of the low mass diskless candidate members in the V-I vs. J-K diagram.
However, we find evidence of differential reddening in this region.

Using the V vs. B-V (from the literature) diagram,
and assuming the mean cluster reddening E(B-V)=0.6, we find that the main sequence members
selected in this work are well fitted by a 1.5 Myr isochrone, if a distance of 3.6±0.2 kpc
is adopted. We note that in the MS cluster region, isochrones degenerate and therefore
our distance estimate is independent of the age adopted for MS members. The value of 3.6 kpc
is consistent with the cluster distance we derive by using the main sequence blue envelope
traced in the V vs. V-I diagram by foreground field stars located
at the cluster distance. Therefore, using independent methods and data, we are able to firmly
evaluate the cluster distance for which values between 3250 and 6000 pc are given in the most recent
literature.

Finally, we derived masses and ages of selected candidate members in the optical PMS region down to
about 0.2 M⊙; median ages for diskless and class II stars, are, respectively,
1.4 and 1.6 Myr, with a standard deviation of 1.8 and 2.2 Myr for the two samples.
Few cluster candidate members are older with ages up to about 10 Myr.

If we consider that 1001 of the 1061 class II candidate members are within the Chandra FoV, where we have
selected the diskless candidate members, we find a disk fraction of about 71% of stars. This value
is an upper limit since the diskless member selection is based on the X-ray observations that reach
a mass limit higher than that we can reach with NIR observations used to select class II stars.
However, the disk fraction we find is similar to the 67% value we found in Paper I and in agreement
with the disk fraction found in clusters of similar age (hais01), if we assume for NGC 1893 a median
age of 1.5 Myr. A detailed analysis of the disk fraction as a function of stellar masses and cluster location
will be the subject of a forthcoming paper (Sanz-Forcada et al. 2010, in preparation).

We therefore conclude that, despite its peculiar location in the Galaxy,
NGC 1893 includes a rich population of young stars, with general
properties similar
to those found in young clusters in the solar neighborhood.

Acknowledgements.

Part of this work was financially supported by the PRIN-INAF (P.I. Lanza)
and the EC MC RTN CONSTELLATION (MRTNâCTâ2006â035890).
We thank Donata Randazzo for carefull reading of the paper.

References

Appendix A Photometric calibration

a.1 Dolores

Photometric calibration of DOLORES observations was computed by using
the images of the land92 standard field SA 98. We used the
v, r, i, hα instrumental magnitudes, and considered the V,
R and I
magnitudes of the Johnson-Kron-Cousins photometric system of standard
stars in these fields (ste00); Hα is the magnitude in the
corresponding non-standard filter.
We calculated the transformation coefficients
between the instrumental and standard systems by using the CCDSTD code (stet05)
and the following equations:

v

=

V+A0+A1Q+A2(V−I)+A3(V−I)2+A4XY+

+A5X+A6Y+A7X2+A8Y2

r

=

R+B0+B1Q+B2(R−I)+B3(R−I)2+B4XY+

(3)

+B5X+B6Y+B7X2+B8Y2

i

=

I+C0+C1Q+C2(V−I)+C3(V−I)2+C4XY+

+C5X+C6Y+C7X2+C8Y2

hα

=

Hα+D0+D1Q+D2(R−I)+D3(R−I)2+D4XY+

+D5X+D6Y+D7X2+D8Y2

where Q is the airmass, A0, B0 and C0 are the magnitude
zero points; A1, B1 and C1 are
the extinction coefficients;
A2, B2 and C2 and A3, B3 and C3 are the color terms, and the rest of terms are
related to the geometrical coordinates (X,Y) of the
plate. Stars with magnitude residual values above the 3σ level
were not considered for the coefficient fitting.
Coefficients calculated during calibration are listed in
Table 10. DOLORES extinction terms are set to those
provided by the telescope web site. We
followed the same nomenclature for the Hα filter, but since we have
no standard measurements in this band, we applied the same calibration as
for the R band (therefore Di=Ci) in the case of DOLORES.

Instr.

Date

Filter

Coeff.

Ai, Bi, Ci, Di coefficient indices

0

1

2

3

4

5

6

7

8

D

2007/09/21

V

Ai

-0.874

0.15

0.087

…

…

-0.068

0.138

…

-0.045

±0.014

±0.007

…

…

±0.006

±0.021

…

±0.010

D

”

R

Bi

-0.823

0.11

-0.002

…

…

-0.072

-0.007

…

…

”

±0.008

±0.009

…

…

±0.004

±0.003

…

…

D

”

I

Ci

-0.531

0.07

-0.035

…

…

0.420

0.393

-0.191

-0.181

”

±0.020

±0.006

…

…

±0.026

±0.017

±0.010

±0.008

D

2007/10/18

V

Ai

-1.039

0.15

0.103

…

…

-0.007

0.015

…

-0.006

±0.004

±0.002

…

…

±0.002

±0.007

…

±0.003

D

”

R

Bi

-1.100

0.11

0.015

…

…

0.013

0.011

…

…

”

±0.003

±0.004

…

…

±0.002

±0.002

…

…

D

”

I

Ci

-0.572

0.07

0.018

…

…

0.237

0.224

-0.081

-0.117

”

±0.009

±0.004

…

…

±0.019

±0.010

±0.011

±0.005

D

2007/11/14

V

Ai

-1.019

0.15

0.090

…

…

-0.014

0.072

…

-0.026

±0.013

±0.004

…

…

±0.016

±0.011

…

±0.005

D

”

R

Bi

-1.050

0.11

-0.007

…

…

0.043

0.002

-0.026

…

”

±0.005

±0.004

…

…

±0.008

±0.002

±0.003

…

D

”

I

Ci

-0.537

0.07

-0.019

…

…

0.307

0.239

-0.136

-0.128

”

±0.010

±0.004

…

…

±0.014

±0.010

±0.006

±0.005

C

2007/10/11

V

Ai

2.482

0.118

-0.022

0.015

-0.026

-0.236

-0.127

0.139

0.086

±0.019

±0.004

±0.015

±0.005

±0.011

±0.022

±0.023

±0.010

±0.012

C

”

R

Bi

1.820

0.045

0.030

-0.056

-0.147

0.261

0.151

…

…

”

±0.042

±0.011

±0.049

±0.031

±0.061

±0.082

±0.091

…

…

C

”

I

Ci

2.467

0.051

-0.082

-0.015

-0.039

0.206

0.344

-0.132

-0.208

”

±0.021

±0.005

±0.009

±0.004

±0.025

±0.030

±0.040

±0.021

±0.022

C

”

Hα

Di

-1.344

1.088

0.340

…

0.063

-0.333

-0.384

0.205

0.238

”

±0.027

±0.323

±0.036

…

±0.014

±0.028

±0.029

±0.012

±0.016

C

2008/01/05

V

Ai

2.642

…

0.063

-0.012

-0.018

0.190

0.118

-0.094

-0.098

±0.012

…

±0.011

±0.004

±0.008

±0.014

±0.013

±0.007

±0.007

C

”

R

Bi

1.940

…

0.038

-0.037

-0.010

0.201

0.129

-0.099

-0.131

”

±0.010

…

±0.014

±0.008

±0.007

±0.012

±0.011

±0.005

±0.006

C

”

I

Ci

4.497

…

-0.108

-0.006

-0.222

-0.200

0.233

0.174

-0.043

”

±0.021

…

±0.011

±0.003

±0.015

±0.027

±0.024

±0.013

±0.013

C

”

Hα

Di

2.948

…

-0.373

0.374

0.027

-0.266

-0.594

0.162

0.347

”

±0.105

…

±0.154

±0.127

±0.060

±0.130

±0.100

±0.052

±0.059

C

2008/01/09

Hα

Di

1.724

…

0.045

-0.013

0.118

0.322

-0.266

-0.176

0.028

±0.045

…

±0.051

±0.035

±0.026

±0.049

±0.050

±0.019

±0.024

Notes:aDOLORES observations in Hα have same coefficients as for the R filter

Table 10: Coefficients of the transformations to the standard system, given in the
Eq.s A.1, for
each filter and night for DOLORES (D)a and CAFOS (C)

a.2 Cafos

We tried to calibrate the CAFOS observations following the same procedure
as for DOLORES, using the site extinction terms as provided by
sanchez07. However we noticed substantial discrepancies in the
final magnitudes calculated for CAFOS when compared to DOLORES
results. The variable atmospheric conditions detected during the
observations, also noted in the seeing, could be responsible of the
inconsistencies. We decided to use the DOLORES
photometry to calibrate the CAFOS fields: we selected the local
standards used in DOLORES fields that are also common to the CAFOS
FoV, and used a total of 417 CAFOS counterparts as standards for the
calibration, so we calibrated the CAFOS observations in the
same photometric system of DOLORES. The second and third nights of CAFOS
observations also required to set the “cloud” variable in the CCDSTD
setting. As explained by ste00 this
variable allows
to consider different atmospheric conditions (thin clouds) during the
night, setting
all the extinction terms to 0 in this case (Table 10),
and fitting the zero points and the other terms.
CAFOS observations used the DOLORES standard stars falling in the field to
calibrate the Hα observations, with Di coefficients as
listed in Table 10.

a.3 Nics

We used the standalone DAOPHOT II/ALLSTAR code
(stet87) to obtain the instrumental photometry for the
combined images obtained with SNAP for each filter (JHK) and for each of the 16 fields
observed around the cluster NGC 1893. For most of the images we performed the PSF photometry using
the Moffat function with β=2.5, typically used to model stellar profiles as an analytic first
approximation to the PSF, while for 6 images
we used the more complex Penny function in order to take into account the elongated shape of the
PSF likely due to some aberration. In addition we treated the variable PSF by using
the DAOPHOT option that considers a PSF which varies quadratically with position in the frame.

Since the PSF photometry is relative to the model stellar profile of a given frame,
we needed to derive the aperture correction to the instrumental photometry; to this aim
we performed aperture photometry
at different radii on a sample of isolated and relatively bright stars from which we
derived growth curves with the DAOGROW code (stet90). The aperture correction has been computed using the selected stars as the median of the
difference between the PSF magnitudes and the aperture magnitudes obtained at the radius including all
the stellar flux.

In order to discard false identifications due to the spots either
of very bright objects or just falling at the
edge of the images, we selected the list of objects detected
by considering only those following the
typical exponential profile of the magnitude errors. Then for each of the 16 observed fields,
we merged with DAOMATCH/DAOMASTER (stet87) the three lists with JHK magnitudes in order to
have a single list with objects having at least two among the JHK magnitudes.

The NICS filters used for our observations (Js, H, K’) are those of the Mauna
Kea Observatories (MKO) near-infrared filter set (ghin02) and therefore we
calibrate our catalog in the MKO photometric system. To this aim we use as standard
stars, the 2MASS counterparts falling in our fields with PH_QUAL flag equal to ’AAA’ and
CC_FLG flag equal to ’000’. We first converted the 2MASS
catalog in the MKO system by using the inverted transformations given in the 2MASS
web page444http://www.ipac.caltech.edu/2mass/releases/allsky/doc/sec6_4b.html
maintened by J. Carpenter (see also legg06).

By using instrumental magnitudes for the standard stars, that we indicate as j, h, k and those of the 2MASS catalog in the MKO system,
that we indicate as JMKO, HMKO, KMKO
we performed for each field a linear fit of the magnitude differences as a function of the colors as
in the following equations:

(j−JMKO)=A0+A1(j−h)

(j−JMKO)=B0+B1(j−k)

(h−HMKO)=C0+C1(j−h)

(4)

(h−HMKO)=D0+D1(h−k)

(k−KMKO)=E0+E1(j−k)

(k−KMKO)=F0+F1(h−k).

From the linear fit we derived the zero points, indicated by coefficients with subscript 0
and the color terms, indicated by coefficients with subscript 1 given in Table 11.